1. Field of the Invention
This invention relates to nuclear well logging techniques, wherein a sonde is lowered in a well or borehole and carries out spectral measurements from which are derived information about the composition and/or the physical structure of the earth formation surrounding the well, or the borehole fluid, or the annulus including casing and cement located between the borehole wall and the formation. The invention can also be applied to logging while drilling technique (usually referred to as "LWD") where measurement devices, disposed close to the bottom of a borehole drilling system, perform measurements while the borehole is drilled.
2. Related Art
The nuclear logging or LWD techniques involve analysis of either energy spectra or counts of gamma rays or neutrons representative of atoms of elements in earth formations. Different characteristics of interest of the formation may be derived from such analysis depending on the type of technique involved.
For example, the formation density may be derived from the analysis of the Compton scattering of gamma rays emitted in the borehole by electrons of atoms in the formation, as shown in U.S. Pat. No. 3,900,733 to Seeman and asigned to the assignee of the present application.
Another characteristic, porosity, can be derived from the density measurements. Alternatively, porosity can be obtained from the neutron porosity log technique, such as described in U.S. Pat. No. 4,816,674 assigned to assignee of the present application, wherein the formation is irradiated with fast neutrons which, after a few collisions with atoms, are slowed down to thermal energies (i.e. around 0.025 eV). The slowing rate of neutrons depends to a large extent on the amount of hydrogen in the formation. Thermalized neutrons diffuse randomly until they are captured by chlorine, hydrogen or silicon nuclei which become excited and emit high energy gamma rays hereafter called "capture gamma rays". The neutron porosity log is based on the detection of either the thermalized neutrons by themselves or the capture gamma rays. The counting rate at the detector increases for decreased hydrogen concentration.
According to a further example, analysis of gamma ray spectra resulting from the capture of thermal neutrons, after being decomposed into contributions due to individual atomic elements, usually called "elemental yields", can provide information on lithology. Presence of earth formation elements such as e.g. hydrogen, silicon, calcium, chlorine, sulfur and iron can thus be revealed. An example of capture gamma ray spectra analysis is depicted in U.S. Pat. No. 3,521,064 to Moran et al. A measured gamma ray energy spectrum, representative of a formation of unknown composition, is compared with a composite spectrum constructed from individual laboratory derived standard spectra of the constituents postulated to comprise the formation. The different amounts of the standard spectra (elemental yields) which give the best fit to the measured spectrum, when weighted by each element sensitivity (i.e. the ability of an element to emit gamma rays and those gamma rays to get detected), represent the relative proportion of the constituents of the formation. By appropriate selection of the standards, the proportion of the constituents of interest can be obtained.
A final example is constituted by the spectroscopic method for the determination of hydrocarbon saturation or water saturation, usually called carbon/oxygen method, as depicted e.g. in U.S. Pat. No. 4,937,446 to Roscoe, Stoller and McKeon, assigned to the assignee of the present application.
The trend in the logging industry has always been to combine as much as possible the measurements made in the borehole with information obtained from other sources, e.g. from other logs run in the borehole or core analysis. Such information is hereafter referred to as "prior information".
However, the prior information is usually combined with the actual measurements late in the interpretation process. As a matter of fact, the more independent the prior information is from the actual measurements, the later the prior information is included in the process of the data. Thus, the known methods do not allow the full benefit to be derived from the prior information.
Furthermore, the nuclear measurements are statistical by nature, and thus there is a permanent need for improving the statistical precision of the nuclear measurements.